The NFκB transcription factor was originally identified as a protein complex consisting of a DNA binding subunit and an associated protein. There are five family members in mammals: RelA, c-Rel, RelB, NF-κB1 (p105/p50), and NF-κB2 (p100/p52). Both p105 and p100 are proteolytically processed by the proteasome to produce p50 and p52, respectively. A second protein designated p52 (previously referred to as p49) has been identified that can act as an alternative NFκB subunit. Rel B does not bind with high affinity to NFκB sites, but heterodimers between Rel B and p50 bind with an affinity comparable to that of p50 NFκB homodimers.
